This invention relates generally to optical elements and more particularly to impact protection and antireflection coating of optical elements.
As is known in the art, optical imaging systems generally include one or more externally mounted optical elements which shield the remainder of the imaging system from an external environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element such as a window or dome is generally mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humidity, corrosive, and abrasive environments. Prolonged exposure to these environments generally degrades the optical and physical characteristics of the material of the external optical element. Generally, however, the most severe environmental exposure encountered by such external optical elements appear to be high velocity water droplet impact which occurs when an airborne system is flown through a rain field.
This problem of water droplet impact is more generally referred to in the art as rain erosion. During flight through a rain field, water droplets from a rain field impinge upon the surface of the external element producing subsurface fractures even at subsonic velocities. For very brittle materials, these subsurface fractures are initiated at pre-existing microflaws lying near or at the surface of the optical element. Rain erosion damage to such optical elements occurs prior to any significant removal of material. The mere propagation of these pre-existing microflaws is sufficient to damage the optical element. In particular, these microflaws are propagated through the optical element by the tensile component of a surface stress wave created at the time of impact with the water droplet. Once formed, the continued propagation of a subsurface fracture through the optical element will often produce large cracks in the optical element. In the region of the cracks scattering and refraction of incident IR energy occurs producing increased internal reflections and IR energy losses. With a significant number of such cracks, the transmissivity of the optical element is severely reduced. Furthermore, as cracks propagate through the optical element, catastrophic failure of the element may occur. When the optical element shatters or breaks, the remaining optical elements of IR imaging system are exposed to the external environment resulting in potential catastrophic damage to the imaging system.
Typically, materials which offer the best mechanical durability and optical performance for infrared imaging systems, such as long wavelength infrared energy (LWIR) particularly in the 8.0 micron to 12.0 micron infrared band are limited to a relatively small number. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride, and cadmium telluride. Ternary sulfides having the formula MLn.sub.2 S.sub.4, where M is a group 1 cation, Ln is a lanthanide rare earth series cation and S is the S.sup.-2 sulfide anion, such as calcium lanthanum sulfide are also currently being developed for IR applications particularly in the 8 to 12 micrometer band. These ternary sulfide materials may provide some improvements in durability but even these materials are susceptible to the environmental exposures mentioned above. Generally, al of the aforementioned materials are relatively brittle and have a relatively low resistance to damage, particularly damage sustained during high velocity water droplet impact.
It is also known in the art that optical energy incident upon a surface of an optical element will result in reflection of such energy at such surface if the index refraction of the material comprising the optical element is significantly different than the index of refraction of the medium from which the energy originates. Generally, for airborne systems, the originating medium is air having an index of refraction of about 1. Accordingly, it is standard practice in the optical industry to provide coatings of materials of appropriate refractive index over the incidence surface of the optical element to reduce such reflection losses. At the deposited thicknesses which are generally related to a fraction of an optical wavelength, these coatings are transparent in the IR band. However, heretofore, such optical coatings have served only to reduce reflection losses caused by a mismatch in refractive indices and have not served to increase the impact resistance of the optical element in any significant manner.
It is also known in the art that a layer of hard carbon, that is a carbon layer having quasi-diamond bonds and substantial optical transparency, when provided over germanium, provides limited protection to germanium optical elements from impact damage caused by rain erosion. Hard carbon coatings on germanium are described in an article entitled "Liquid Impact Erosion Mechanisms and Transparent Materials" by J. E. Fields et al, Final Reports Sept. 30, 1982 to Mar. 31, 1983, Contract No. AFOSR-78-3705-D, Report No. AFWAL-TR-83-4101. The hard carbon materials have not successfully adhered directly to other IR materials such as zinc sulfide and zinc selenide. Furthermore, hard carbon coatings, even on germanium as mentioned in the article, are susceptible to debonding during high velocity water droplet impact. It was theorized in that article that the sheering force resulting from radial outflow during water droplet impact causes the debonding of the hard carbon coating from the germanium layer. This phenomenon of debonding is believed to significantly increase as the thickness of the hard carbon layer is increased. Therefore, while thicker hard carbon coating layers should result in further impact protection for the optical element, in fact these thicker layers are more susceptible because of the aforementioned debonding problem. A further problem with hard carbon is that the index of refraction of hard carbon is about 2.45, substantially higher than index of refraction of many of the aforementioned optical materials such as zinc sulfide and zinc selenide. Accordingly, if an optical element is coated with a hard carbon coating, reflection losses at the incident surface of the optical element would be higher than if the optical element was not coated.
It is desirable, therefore, to provide an optical element having a high degree of durability and resistance to environmental exposures particularly high velocity water droplet impact and having enhanced optical properties within the wavelength band of 8 micrometers to 12 micrometers, particularly at certain regions within said wavelength band.
It is particularly desirable to provide an impact protection, antireflection layer to protect particularly brittle materials such as zinc sulfide and zinc selenide at least over the 8 .mu.m to 12 .mu.m band.